An above-knee amputation is a highly debilitating condition. Transfemoral amputees exert as much as twice the energy of their counterparts with fully intact lower limbs when walking on level ground. Tasks such as climbing stairs or standing up from a seated position are exceedingly more difficult for transfemoral amputees.
Passive prosthetic knees provide some support for walking. Some passive prosthetic knees incorporate microprocessor devices to intelligently control the compliance of the knee. A passive prosthetic knee does not provide additional power to the knee beyond the power provided by its user. A user of a passive prosthetic knee must compensate to adapt to the lost knee power during walking and while performing other tasks. For instance, a user of a passive prosthetic knee may lead with his or her sound limb when climbing curbs or stairs. As another example, users of passive prosthetic knees will “side step” up or down ramps. Some users of passive prosthetic knees do not have the strength to perform these tasks by compensating for the lack of power from the knee. In particular, elderly transfemoral amputees generally do not have the strength to use a passive prosthetic knee. As a result, many elderly transfemoral amputees, particularly those without family support, live in nursing homes rather than their own homes.
A powered prosthetic knee can provide a user with lost functionality by providing power similar to power provided by a biological knee. One commercially available powered prosthetic knee is the Power Knee by Össur (Reykjavik, Iceland), which uses a motor to provide the power. A powered prosthetic knee can give more functionality to its user. However, in the prior art, the added functionality comes at the cost of a prosthetic knee with greater weight. For example, the Power Knee weighs 3.19 kg (7.1 lbs), more than twice that of most passive prosthetic knees. The extra weight in a powered prosthetic knee is in the motor, transmission, and battery needed to provide sufficient power over a reasonable period of time. The user must carry this extra weight when walking, climbing stairs, or performing other tasks.
To date, powered prostheses have not improved the efficiency of gait. We believe this is due to the extra weight of powered prostheses in the prior art, including weight from the motor, transmission, and battery. Powered lower limb prostheses improve a user's ability to climb stairs, but most people do not climb stairs during much of the day. The ability to climb stairs and perform other tasks that require a powered prosthetic knee is important to accomplish many activities of daily living, but such tasks make up a relatively small portion of a user's daily mobility needs.